U.S. patent number 6,855,450 [Application Number 09/909,846] was granted by the patent office on 2005-02-15 for proton exchange membrane electrochemical cell system.
This patent grant is currently assigned to Proton Energy Systems, Inc.. Invention is credited to Mark E. Dristy, Trent M. Molter.
United States Patent |
6,855,450 |
Molter , et al. |
February 15, 2005 |
Proton exchange membrane electrochemical cell system
Abstract
A compression member for an electrochemical cell stack is
provided. The compression member includes a first surface including
a plurality of raised portions, a second surface including a
substantially flat surface, and an edge defined by the first
surface and the second surface. The plurality of raised portions is
aligned to define a plurality of receiving areas. The plurality of
raised portions and the plurality of receiving areas are configured
such application of an axial compressive force spreads the
plurality of raised portions into the plurality of receiving areas.
The edge includes a portion configured to receive an
electrochemical cell terminal therethrough. The compression member
is formed of electrically non-conductive materials.
Inventors: |
Molter; Trent M. (Glastonbury,
CT), Dristy; Mark E. (Kutztown, PA) |
Assignee: |
Proton Energy Systems, Inc.
(Rocky Hill, CT)
|
Family
ID: |
22819620 |
Appl.
No.: |
09/909,846 |
Filed: |
July 20, 2001 |
Current U.S.
Class: |
429/458; 204/258;
429/512; 429/469; 429/508 |
Current CPC
Class: |
H01M
8/248 (20130101); H01M 8/246 (20130101); C25B
9/73 (20210101); H01M 8/025 (20130101); H01M
8/0273 (20130101); H01M 8/247 (20130101); H01M
8/0247 (20130101); H01M 8/0202 (20130101); H01M
2300/0082 (20130101); Y02E 60/50 (20130101); H01M
8/241 (20130101); Y02E 60/36 (20130101) |
Current International
Class: |
C25B
1/10 (20060101); C25B 1/00 (20060101); C25B
9/00 (20060101); H01M 2/08 (20060101); H01M
8/02 (20060101); H01M 008/02 (); C25B 009/00 () |
Field of
Search: |
;429/38,39
;204/252,253,257,258 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kalafut; Stephen J.
Attorney, Agent or Firm: Cantor Colburn LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Patent
Application Ser. No. 60/219,526 filed Jul. 20, 2000, the entire
content of which is incorporated herein by reference.
Claims
What is claimed is:
1. A frame member for an electrochemical cell stack, comprising: an
outer periphery; an inner periphery; a surface defined between said
outer periphery and said inner periphery; a fluid port defined
axially through said surface, said fluid port having a length along
said inner and said outer peripheries; a plurality of fluid
manifolds defined in said surface, each of said plurality of fluid
manifolds defining a fluid flow channel between said fluid port and
said inner periphery, and at least one of said plurality of fluid
manifolds extending along said inner periphery a distance beyond
said length of said fluid port; and a gap defined in said inner
periphery, said gap being radially offset along said inner
periphery in a first direction from said fluid port, said gap being
in fluid communication with said inner periphery, said at least one
of said plurality of fluid manifolds defining a fluid flow channel
between said fluid port and said gap.
2. The frame member of claim 1, further comprising: a second gap
defined is said inner periphery, said second gap being radially
offset along said inner periphery in a second direction from said
fluid port, said second direction being opposite said first
direction, said second gap being in fluid communication with said
inner periphery, a different one of said plurality of fluid
manifolds defining a fluid flow channel between said fluid port and
said second gap.
3. The frame member of claim 1, wherein said plurality of fluid
manifolds and said gap enhance fluid distribution across a flow
field defined within said inner periphery.
4. A frame member for an electrochemical cell stack, comprising: an
outer periphery; an inner periphery; a surface defined between said
outer periphery and said inner periphery; a fluid port defined
axially through said surface, said fluid port having a length along
said inner and said outer peripheries; a plurality of fluid
manifolds defined in said surface, each of said plurality of fluid
manifolds defining a fluid flow channel extending radially from
said fluid port to said inner periphery, and at least one of said
plurality of fluid manifolds extending along said inner periphery a
distance beyond said length of said fluid port; and a protector lip
provided at said inner periphery, said protector lip being adapted
to prevent a flow field of the electrochemical cell stack from
extruding into said plurality of fluid manifolds.
5. The frame member of claim 4, wherein said protector lip is
integral with the flame member.
6. A frame member for an electrochemical cell stack, comprising: an
outer periphery; an inner periphery; a surface defined between said
outer periphery and said inner periphery; a fluid port defined
axially through said surface, said fluid port having a length along
said inner and said outer peripheries; a plurality of fluid
manifolds defined in said surface, each of said plurality of fluid
manifolds defining a fluid flow channel between said fluid port and
said inner periphery, and at least one of said plurality of fluid
manifolds extending along said inner periphery a distance beyond
said length of said fluid port; and a gap disposed in said inner
periphery, said gap extending about said inner periphery and being
radially offset along said inner periphery in a first direction
from said fluid port, said at least one of said plurality of fluid
manifolds defining a fluid flow channel between said fluid port and
said channel.
7. The frame member of claim 6, further comprising ridges disposed
on said surface proximate to said fluid port.
8. In a hydrogen generating system including a water source, an
electrochemical cell stack, an electrical source, a first
separator, a second separator, a dryer, a controller, and a
ventilation system, wherein the improvement comprises: a first flow
field within said electrochemical cell stack between a first
electrode and a separator, said first flow field being surrounded
in the radial direction by a first frame, and a second flow field
between a second electrode and a separator surrounded in the radial
direction by a second frame, a boundary defined between an inside
edge of said first frame and an outside edge of said flow field,
wherein said boundary is configured with gaps in fluid
communication with one or more manifolds.
9. A frame member for an electrochemical cell stack, comprising: an
outer periphery; an inner periphery; a surface defined between said
outer periphery and said inner periphery; a fluid port defined
axially through said surface, said fluid port having a length along
said inner and said outer peripheries; ridges disposed on said
surface proximate to said fluid port; and a plurality of fluid
manifolds defined in said surface, each of said plurality of fluid
manifolds defining a fluid flow channel extending radially from
said fluid port to said inner periphery, and at least one of said
plurality of fluid manifolds extending along said inner periphery a
distance beyond said length of said fluid port.
Description
BACKGROUND
The present disclosure relates to electrochemical cells, and in
particular to features of proton exchange membrane electrochemical
cell systems.
Electrochemical cells are energy conversion devices, usually
classified as either electrolysis cells or fuel cells. A proton
exchange membrane electrolysis cell can function as a hydrogen
generator by electrolytically decomposing water to produce hydrogen
and oxygen gas, and can function as a fuel cell by
electrochemically reacting hydrogen with oxygen to generate
electricity. Referring to FIG. 1, which is a partial section of a
typical anode feed electrolysis cell 100, process water 102 is fed
into cell 100 on the side of an oxygen electrode (anode) 116 to
form oxygen gas 104, electrons, and hydrogen ions (protons) 106.
The reaction is facilitated by the positive terminal of a power
source 120 electrically connected to anode 116 and the negative
terminal of power source 120 connected to a hydrogen electrode
(cathode) 114. The oxygen gas 104 and a portion of the process
water 108 exit cell 100, while protons 106 and water 110 migrate
across a proton exchange membrane 118 to cathode 114 where hydrogen
gas 112 is formed.
Another typical water electrolysis cell using the same
configuration as is shown in FIG. 1 is a cathode feed cell, wherein
process water is fed on the side of the hydrogen electrode. A
portion of the water migrates from the cathode across the membrane
to the anode where hydrogen ions and oxygen gas are formed due to
the reaction facilitated by connection with a power source across
the anode and cathode. A portion of the process water exits the
cell at the cathode side without passing through the membrane.
A typical fuel cell uses the same general configuration as is shown
in FIG. 1. Hydrogen gas is introduced to the hydrogen electrode
(the anode in fuel cells), while oxygen, or an oxygen-containing
gas such as air, is introduced to the oxygen electrode (the cathode
in fuel cells). Water can also be introduced with the feed gas. The
hydrogen gas for fuel cell operation can originate from a pure
hydrogen source, hydrocarbon, methanol, or any other hydrogen
source that supplies hydrogen at a purity suitable for fuel cell
operation (i.e., a purity that does not poison the catalyst or
interfere with cell operation). Hydrogen gas electrochemically
reacts at the anode to produce protons and electrons, wherein the
electrons flow from the anode through an electrically connected
external load, and the protons migrate through the membrane to the
cathode. At the cathode, the protons and electrons react with
oxygen to form water, which additionally includes any feed water
that is dragged through the membrane to the cathode. The electrical
potential across the anode and the cathode can be exploited to
power an external load.
In other embodiments, one or more electrochemical cells can be used
within a system to both electrolyze water to produce hydrogen and
oxygen, and to produce electricity by converting hydrogen and
oxygen back into water as needed. Such systems are commonly
referred to as regenerative fuel cell systems.
Electrochemical cell systems typically include one or more
individual cells arranged in a stack, with the working fluids
directed through the cells via input and output conduits formed
within the stack structure. The cells within the stack are
sequentially arranged, each including a cathode, a proton exchange
membrane, and an anode (hereinafter "membrane electrode assembly",
or "MEA"). Each cell typically further comprises a first flow field
in fluid communication with the cathode and a second flow field in
fluid communication with the anode. The MEA may be supported on
either or both sides by screen packs or bipolar plates disposed
within the flow fields, and which may be configured to facilitate
membrane hydration and/or fluid movement to and from the MEA.
In order to maintain intimate contact between cell components under
a variety of operational conditions and over long time periods,
uniform compression is applied to the cell components. Thus, while
existing compression in current electrochemical cells are suitable
for their intended purposes, there still remains a need for
improvements, particularly regarding devices and methods for
providing uniform compression to the electrochemical cell.
SUMMARY
The above-described drawbacks and disadvantages are alleviated by a
compression member for an electrochemical cell stack. The
compression member includes a first surface including a plurality
of raised portions, a second surface including a substantially flat
surface, and an edge defined by the first surface and the second
surface. The plurality of raised portions is aligned to define a
plurality of receiving areas. The plurality of raised portions and
the plurality of receiving areas are configured such that
application of an axial compressive force spreads the plurality of
raised portions into the plurality of receiving areas. The edge
includes a portion configured to receive an electrochemical cell
terminal therethrough. The compression member is formed of
electrically non-conductive materials.
An electrically conductive bus plate for an electrochemical cell
stack is provided. The bus plate includes a substantially planar
portion defining an edge and a terminal portion extending from the
edge. The terminal portion includes a first portion and a second
portion. The first portion is substantially perpendicular to the
substantially planar portion, while the second portion is angled
with respect to the first portion toward the substantially planar
portion.
An electrochemical cell stack is provided. The electrochemical cell
stack includes a first endplate, a second endplate, an
electrochemical cell, a first conductor, and a second conductor.
The first endplate has one fluid passage for a water feed, one
fluid passage for an oxygen output, and one fluid passage for a
hydrogen output. The electrochemical cell is disposed between a
first separator and a second separator. The electrochemical cell
includes a first electrode in electrical communication with the
first separator, a second electrode in electrical communication
with the second separator, and a membrane layer between the first
electrode and the second electrode. The first electrode is in fluid
communication with the fluid passage for the water feed and the one
fluid passage for the oxygen output. The second electrode is in
fluid communication with the one fluid passage for the hydrogen
output. The first conductor is accessible through the first
endplate and is in electrical communication with the first
separator. Similarly, the second conductor is accessible through
the second endplate and is in electrical communication with the
second separator.
An electrochemical cell stack is provided. The electrochemical cell
stack includes a first endplate, a second endplate, an
electrochemical cell, a first non-conductive compression member,
and a second non-conductive compression member. The electrochemical
cell is between a first separator and a second separator. The
electrochemical cell includes a first electrode in electrical
communication with the first separator, a second electrode in
electrical communication with the second separator, and a membrane
layer between the first electrode and the second electrode. The
first non-conductive compression member is between the first
separator and the first endplate. Similarly, the second
non-conductive compression member is between the second separator
and the second endplate. The first electrode is accessible through
the first non-conductive compression member and the first endplate,
while the second electrode is accessible through the second
non-conductive compression member and the second endplate.
A frame member for an electrochemical cell stack is provided. The
frame member includes an outer periphery, an inner periphery and a
surface defined by the peripheries. The frame member also includes
a fluid port defined axially through the surface and a plurality of
fluid manifolds defined in the surface. The fluid port has a length
along the inner and the outer peripheries. Each of the plurality of
fluid manifolds defines a fluid flow channel between the fluid port
and the inner periphery. Moreover, at least one of the plurality of
fluid manifolds extends along the inner periphery a distance beyond
the length of the fluid port.
An improvement in a hydrogen generating system including a water
source, an electrochemical cell stack, an electrical source, a
high-pressure separator, a low-pressure separator, a dryer, a
controller, and a ventilation system is provided. The improvement
includes a first flow field within the electrochemical cell stack
between a first electrode and a separator. The first flow field is
surrounded in the radial direction by a first frame. Similarly, a
second flow field between a second electrode and a separator is
surrounded in the radial direction by a second frame. A boundary is
defined between an inside edge of the first frame and an outside
edge of the flow field, wherein the boundary is configured with
gaps in fluid communication with one or more manifolds.
The above discussed and other features and advantages will be
appreciated and understood by those skilled in the art from the
following detailed description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the drawings, which are meant to be exemplary and
not limiting, and wherein like elements are numbered alike in the
several Figures:
FIG. 1 is a schematic diagram of a partial prior art
electrochemical cell showing an electrochemical reaction;
FIG. 2 is an expanded schematic diagram of a prior art
electrochemical cell;
FIG. 3 is a front view of an exemplary embodiment of an
electrochemical cell stack;
FIG. 4 is one end view of the electrochemical cell stack of FIG.
3;
FIG. 5 is another end view of the electrochemical cell stack of
FIG. 3;
FIG. 6 is a perspective cutaway view of the electrochemical cell
stack of FIG. 3;
FIG. 7 is an exploded view of the electrochemical cell stack of
FIG. 3;
FIG. 8 is a perspective view of an exemplary embodiment of a
manifold;
FIG. 9 is a top view of the manifold of FIG. 8;
FIG. 10 is a section view of the manifold of FIG. 9, taken along
lines 10--10;
FIG. 11 is a section view of the manifold of FIG. 9, taken along
lines 11--11;
FIG. 12 is a perspective view of an exemplary embodiment of an
insulator;
FIG. 13 is a perspective view of an exemplary embodiment of a bus
plate;
FIG. 14 is a side view of the bus plate of FIG. 13;
FIG. 15 is a bottom perspective view of an exemplary embodiment of
a compression member;
FIG. 16 is a top perspective view of the compression member of FIG.
15;
FIG. 17 is a sectional view of the compression member of FIG. 15,
taken along lines 17--17;
FIG. 18 is an exploded view of an exemplary embodiment of a cell
used within the electrochemical cell;
FIG. 19 is a top view of a frame assembly; and
FIG. 20 is an enlarged view of a portion of the frame assembly of
FIG. 19.
DETAILED DESCRIPTION
Disclosed herein are novel methods and apparatus for providing
uniform compression to cell components of electrochemical cells
under a variety of operational conditions and over long time
periods.
Although the disclosure below is described in relation to a proton
exchange membrane electrochemical cell employing hydrogen, oxygen,
and water, various reactants may also be used, including, but not
limited to, hydrogen bromine, oxygen, air, chlorine, and iodine.
Upon the application of different reactants and/or different
electrolytes, the flows and reactions are understood to change
accordingly, as is commonly understood in relation to that
particular type of electrochemical cell.
Referring to FIG. 2, one exemplary embodiment of an electrochemical
cell 200 suitable for operation as an anode feed electrolysis cell,
cathode feed electrolysis cell, fuel cell, or regenerative fuel
cell is schematically shown. Thus, while the discussion below is
directed to an anode feed electrolysis cell, it should be
understood that cathode feed electrolysis cells, fuel cells, and
regenerative fuel cells are also within the scope of the present
invention. Cell 200 is typically one of a plurality of cells
employed in a cell stack as part of an electrochemical cell system.
When cell 200 is used as an electrolysis cell, power inputs are
generally between about 1.48 volts and about 3.0 volts, with
current densities between about 50 A/ft.sup.2 (amperes per square
foot) and about 4,000 A/ft.sup.2. When used as a fuel cells power
outputs range between about 0.4 volts and about 1 volt, and between
about 0.1 A/ft.sup.2 and about 10,000 A/ft.sup.2. The number of
cells within the stack, and the dimensions of the individual cells
is scalable to the cell power output and/or gas output
requirements.
Cell 200 includes a membrane 202 having a first electrode (e.g., an
anode) 204 and a second electrode (e.g., a cathode) 206 disposed on
opposite sides thereof. Flow fields 210, 220, which are in fluid
communication with electrodes 204 and 206, respectively, are
defined generally by the regions proximate to, and bounded on at
least one side by, each electrode 204 and 206 respectively. A flow
field member 228, for example a screen pack or bipolar plate, is
optionally disposed within flow field 220 between electrode 206
and, optionally, a pressure pad separator plate 222. A pressure pad
230 is typically disposed between pressure pad separator plate 222
and a cell separator plate 232. Cell separator plate 232 is
disposed adjacent to pressure pad 230. A frame 224, generally
surrounding flow field 220 and an optional gasket 226, is disposed
between frame 224 and pressure pad separator plate 222 generally
for enhancing the seal within the reaction chamber defined on one
side of cell 200 by frame 224, pressure pad separator plate 222 and
electrode 206. Gasket 236 is optionally disposed between pressure
pad separator plate 222 and cell separator pad 232 enclosing
pressure pad 230.
Another flow field member 218 is optionally disposed in flow field
210. A frame 214 generally surrounds flow field member 218, a cell
separator plate 212 is disposed adjacent flow field member 218
opposite oxygen electrode 204, and a gasket 216 is disposed between
frame 214 and cell separator plate 212, generally for enhancing the
seal within the reaction chamber defined by frame 214, cell
separator plate 212 and the oxygen side of membrane 202. The cell
components, particularly cell separator plates 212, 232, frames
214, 224, and gaskets 216, 226, and 236 are formed with the
suitable manifolds or other conduits as is conventional.
Membrane 202 comprises electrolytes that are preferably solids or
gels under the operating conditions of the electrochemical cell.
Useful materials include proton conducting ionomers and ion
exchange resins. Useful proton conducting ionomers include
complexes comprising an alkali metal salt, alkali earth metal salt,
a protonic acid, or a protonic acid salt. Useful complex-forming
reagents include alkali metal salts, alkaline metal earth salts,
and protonic acids and protonic acid salts. Counter-ions useful in
the above salts include halogen ion, perchloric ion, thiocyanate
ion, trifluoromethane sulfonic ion, borofluoric ion, and the like.
Representative examples of such salts include, but are not limited
to, lithium fluoride, sodium iodide, lithium iodide, lithium
perchlorate, sodium thiocyanate, lithium trifluoromethane
sulfonate, lithium borofluoride, lithium hexafluorophosphate,
phosphoric acid, sulfuric acid, trifluoromethane sulfonic acid, and
the like. The alkali metal salt, alkali earth metal salt, protonic
acid, or protonic acid salt is complexed with one or more polar
polymers such as a polyether, polyester, or polyimide, or with a
network or cross-linked polymer containing the above polar polymer
as a segment. Useful polyethers include polyoxyalkylenes, such as
polyethylene glycol, polyethylene glycol monoether, and
polyethylene glycol diether; copolymers of at least one of these
polyethers, such as poly(oxyethylene-co-oxypropylene) glycol,
poly(oxyethylene-co-oxypropylene) glycol monoether, and
poly(oxyethylene-co-oxypropylene) glycol diether; condensation
products of ethylenediamine with the above polyoxyalkylenes; and
esters, such as phosphoric acid esters, aliphatic carboxylic acid
esters or aromatic carboxylic acid esters of the above
polyoxyalkylenes. Copolymers of, e.g., polyethylene glycol with
dialkylsiloxanes, maleic anhydride, or polyethylene glycol
monoethyl ether with methacrylic acid are known in the art to
exhibit sufficient ionic conductivity to be useful.
Ion-exchange resins useful as proton conducting materials include
hydrocarbon- and fluorocarbon-type resins. Hydrocarbon-type
ion-exchange resins include phenolic resins, condensation resins
such as phenol-formaldehyde, polystyrene, styrene-divinyl benzene
copolymers, styrene-butadiene copolymers,
styrene-divinylbenzene-vinylchloride terpolymers, and the like,
that are imbued with cation-exchange ability by sulfonation, or are
imbued with anion-exchange ability by chloromethylation followed by
conversion to the corresponding quaternary amine.
Fluorocarbon-type ion-exchange resins include hydrates of
tetrafluoroethylene-perfluorosulfonyl ethoxyvinyl ether or
tetrafluoroethylene-hydroxylated (perfluoro vinyl ether)
copolymers. When oxidation and/or acid resistance is desirable, for
instance, at the cathode of a fuel cell, fluorocarbon-type resins
having sulfonic, carboxylic and/or phosphoric acid functionality
are preferred. Fluorocarbon-type resins typically exhibit excellent
resistance to oxidation by halogen, strong acids and bases. One
family of fluorocarbon-type resins having sulfonic acid group
functionality is NAFION.TM. resins (commercially available from E.
I. du Pont de Nemours and Company, Wilmington, Del.).
Electrodes 204 and 206 comprise catalyst suitable for performing
the needed electrochemical reaction (i.e., electrolyzing water to
produce hydrogen and oxygen). Suitable metals from which electrodes
can be fabricated include, but are not limited to, platinum,
palladium, rhodium, carbon, gold, tantalum, tungsten, ruthenium,
iridium, osmium, alloys of at least one of the foregoing catalysts,
and the like. However, while certain catalysts are specifically
listed, it is contemplated that other catalysts capable of
electrolyzing water and producing hydrogen (in the case of
electrolysis cell operation) and/or capable of breaking down
hydrogen into ions (in the case of fuel cell operation) are
suitable with the electrode structure generally described. A
preferred catalyst is platinum or palladium. Electrodes 204 and 206
may be created by layering or pressing electrode catalyst provided
in a planar form on membrane 202. Both techniques are known in the
art.
Flow field members 218, 228 support membrane 202, allow the passage
system fluids to promote hydration of cell components, and
preferably are electrically conductive, and may be, for example,
screen packs or bipolar plates. The screen packs include one or
more layers of perforated sheets or a woven mesh formed from metal
or strands. These screens are typically fabricated of metals that
include, for example, niobium, zirconium, tantalum, titanium,
carbon steel, stainless steel, nickel, cobalt, and alloys thereof.
Bipolar plates are commonly perforated structures through which
fluid communication can be maintained. Materials from which the
bipolar plates can be fabricated include, but are not limited to,
fibrous carbon or fibrous carbon impregnated with
polytetrafluoroethylene or PTFE (commercially available under the
trade name TEFLON.RTM. from E. I. du Pont de Nemours and
Company).
Pressure pad 230 provides even compression between cell components,
is electrically conductive, and therefore generally comprises a
resilient member, preferably an elastomeric material, together with
a conductive material. Suitable elastomeric materials include, but
are not limited to silicones, such as, for example,
fluorosilicones; fluoroelastomers, such as KALREZ.RTM.
(commercially available from E. I. du Pont de Nemours and Company),
VITON.RTM. (commercially available from E. I. du Pont de Nemours
and Company), and FLUOREL.RTM. (commercially available from
Minnesota Mining and Manufacturing Company, St. Paul, Minn.); and
combinations thereof. The electrically conductive material is
preferably compatible with the system fluids and membrane 202.
Suitable electrically conductive materials include, but are not
limited to, conductive metals and alloys and superalloys thereof,
for example niobium; zirconium; tantalum; titanium; niobium; iron
and iron alloys, for examples steels such as stainless steel;
nickel and nickel alloys such as HASTELLOY7 (commercially available
from Haynes International, Kokomo, Ind.); cobalt and cobalt
superalloys such as ELGILOY7 (commercially available from
Elgiloy.RTM. Limited Partnership, Elgin, Ill.) and MP35N7
(commercially available from Maryland Speciality Wire, Inc., Rye,
N.Y.); hafnium, and tungsten, among others, with titanium being
preferred because of its strength, durability, availability, low
cost, ductility, low density, and its compatibility with the
electrochemical cell environment. Conductive carbon is also often
used. In one embodiment, the electrically conductive material
comprises a plurality of VITON.RTM. cords woven or stitched into a
conductive carbon cloth substrate. Pressure pad 230 is optionally
configured to allow passage of water or system gases.
It has been discovered that improvements in the construction and
operation of electrochemical cell 200 are found by providing a
compression system at both ends of the cell, thereby allowing
operation of the electrochemical cell system at cell pressures from
atmospheric pressure up to about 100 pounds per square inch (psi),
preferably up to about 150 psi, more preferably about 250 psi, even
more preferably about 500 psi, and most preferably up to about
1,000 psi or greater. Moreover, it has been determined that
improvements in the cost and ease of manufacture of electrochemical
cell 200 are found by providing separating the electrically
conductive functions and compression function of the compression
system. Additionally, it has been determined that further
improvements in the cost and ease of manufacture of electrochemical
cell 200 are found by providing three port frames within the
cell.
Referring now to FIGS. 3-6, an exemplary embodiment of an
electrochemical cell stack 300 is illustrated. Cell stack 300,
illustrated in FIG. 3, comprises a cell assembly 302, shown for
ease of description as one cell. Cell stack 300 including more than
one cell assembly 302 having separators disposed therebetween is
considered within the scope of the present invention.
Cell assembly 302 is arranged between a first endplate 304 and a
second endplate 306. Endplates 304 and 306 are formed of any
suitable material, such as but not limited to carbon steel,
stainless steel, titanium, aluminum, graphite, ceramics, and the
like. Cell stack 300 includes at least one mounting bracket 308 for
overall securement of the cell stack.
Fittings 310, 312, and 314 are included on first endplate 304,
wherein fitting 310 accepts suitable tubing for feed water (not
shown) from outside cell stack 300, fitting 312 accepts suitable
tubing for water and oxygen output (not shown), and fitting 314
accepts suitable tubing for hydrogen output (not shown). Fittings
310, 312, and 314 are formed of any suitable material, such as but
not limited to carbon steel, stainless steel, titanium, aluminum
and the like.
To facilitate electrical connection, a pair of electrical terminals
316 and 318 extends from first endplate 304 and second endplate
306, respectively. In one embodiment, terminals 316 and 318 extend
axially from endplates 304 and 306. Terminals 316 and 318 are in
electrical contact with separators at the ends of cell assembly 302
(as described further herein) and extend through endplates 304 and
306. Terminals 316 and 318 are formed of any suitable conductive
material, including but not limited to, copper, aluminum, and
alloys of any of the aforementioned conductive materials.
Furthermore, terminals 316 and 318 being plated with conductive
material are considered within the scope of the present invention.
In one embodiment, terminals 316 and 318 are integrally formed with
a bus, as described further herein.
A first shim 320 and a gasket (shown below at 324 with reference to
FIG. 7) are provided between the inside surface of first endplate
304 and one end of cell assembly 302, and a second shim 322 is
provided between the inside surface of second endplate 306 and the
opposite end of cell assembly 302. By way of example, shims 320 and
322 are generally ring shaped plastic components each having a
central opening and fluid passages disposed in the body portion of
the component. Shims 320 and 322 provide electrical insulation from
the electrically conductive members of the cell stack, e.g.,
separators at the ends of cell assembly 302 (as described further
herein).
In one embodiment, as described further herein, bus portions having
terminals 316 and 318 extending therefrom are arranged within the
central openings of the respective shims 320 and 322. In another
embodiment, as described further herein, a pressure pad is included
within the central openings of the respective shims 320 and 322
between (in the axial direction) each of the bus portions and the
respective endplates 304 and 306. Thus, shims 320 and 322 being
configured with suitably dimensioned openings are considered within
the scope of the present invention.
As discussed above, shims 320 and 322 provide electrical insulation
from the electrically conductive members of the cell stack.
Suitable insulation or non-conductive material for shims 320 and
322 include, but are not limited to polyetherimides (e.g.
ULTEM.RTM. 1000 commercially available from General Electric
Company, Pittsfield, Mass.), polycarbonates, polysulfonates, or any
blend or mixture of any of the aforementioned plastics. Gasket 324
is configured to seal the fluid flow. Thus, gasket 324 is
preferably ring shaped and is formed of a sealing material
including but not limited to polytetrafluoroethylene (e.g.,
TEFLON.RTM. or TEFZEL.RTM.).
Cell stack 300 is operated at pressures of up to about 150, or
under higher pressures. For example, the overall pressure of the
system is between about 150 pounds per square inch (psi) and 2500
psi, preferably between about 250 and 1,000 psi. To maintain
structural integrity of cell stack 300, cell assembly 302 and shims
320 and 322 are sandwiched between endplates 304 and 306 and
suitably secured. For example, cell stack 300 is secured with a
plurality of screws 326 and corresponding nuts 328 and washers 330
each through suitable openings in first endplate 304, first shim
320, cell assembly 302, second shim 322, second endplate 306, and a
spring 332. In one embodiment, spring 332 is a disc spring assembly
comprising a plurality of disc springs disposed between a pair of
washers.
In one exemplary embodiment, and referring to FIGS. 7-11, to reduce
potential contamination, first shim 320 is configured with
appropriate bosses 334 that extend into openings in first endplate
304 such that first shim 320 acts as a manifold such that the
system fluids do not contact the materials of first endplate 304.
Thus, bosses 334 provide fluid isolation between the inlet and
outlet fluids with respect to first endplate 304.
By way of example, FIG. 10 illustrates bosses 334 configured for
hydrogen throughput via fitting 314 (not shown), while FIG. 11
illustrates bosses configured for water and oxygen fluid throughput
via fittings 310 and 312, respectively (not shown).
In another exemplary embodiment, and referring to FIGS. 7 and 12,
terminals 316 and 318 are electrically insulated from endplates 304
and 306. FIG. 12 depicts an insulator 336 that fits within mating
slots formed in endplates 304 and 306. A suitable insulator 336 is
a molded component having a slot 337 formed therethrough for
passage of terminals 316 and 318. Further, each insulator 336
preferably includes portion 338 and 340 configured to mate into a
corresponding opening in the respective endplate generally to
maintain positioning of insulator 336 during assembly and
operation, and a portion 342 having a surface 334 configured to be
positioned adjacent to the respective shims 320 and 322.
A suitable non-conductive material for insulators 336 includes, but
is not limited to polyetherimides (e.g. ULTEM.RTM. 1000
commercially available from General Electric Company, Pittsfield,
Mass.), polycarbonates, polysulfonates, or any blend or mixture of
any of the aforementioned materials.
In still another exemplary embodiment, and referring to FIGS. 7,
13, and 14, a bus 346 having terminal 316 extending therefrom is
depicted. A second bus 348 is also depicted in FIG. 7, which has
terminal 318 extending therefrom. Second bus 348 is similar or
identical in structure to bus 346. Bus 346 and/or 348 comprises a
planar portion 350 having terminal 316 extending suitably to
provide access to electrical connection. As depicted in FIG. 14,
bus 346 includes a perpendicular portion 352 extending
substantially normally from planar portion 350, and further having
a portion 354 upon which terminal 316 is disposed.
Portion 354 has a bend of approximately three degrees relative to
portion 352. The bend facilitates formation of sub-assemblies,
e.g., a sub-assembly of bus 346, insulator 336, a compression
member 356 (described further herein), and endplate 304, and a like
sub-assembly on the opposite side of the cell stack. The bent
portion 354 has spring characteristics to hold the sub-assembly
components together during manufacture, transport, or storage.
In certain alternate embodiments, terminal 316 extends
substantially parallel to portion 350, wherein electrical
connection is facilitated proximate to or outside of the radial
edge surface of the stack rather than out of the axial edge of the
stack (e.g., through endplate 304 as described above).
In still another exemplary embodiment illustrated in FIGS. 7 and
15-17, compression member 356 is provided at each end of cell stack
300. Depending on system needs and configurations, compression
members 356 are included on both or either end of stack 300.
Compression members 356 are fabricated from a non-conductive
material that is moldable into the desired shape of the proper
dimensions. With the inclusion of compression members 356, contact
between the electrical bus and the respective separator is
enhanced, thereby promoting lower resistance. In a preferred
embodiment, both compression members 356 are similar or identical
to one another.
Suitable non-conductive materials for compression members 356 are
elastomeric materials, including but not limited to silicone
rubber, fluoroelastomers, such as VITON.RTM. (commercially
available from Dupont de Nemours), terpolymers of ethylene and
propylene, such as EPDM, or any blend or mixture of any of the
aforementioned elastomeric materials.
In one embodiment, as shown in FIG. 17, a ridged surface 360 (as
also shown in FIG. 15) facially opposes the inside surface of the
respective endplate, and a substantially flat surface 362 (as shown
in FIG. 16) facially opposes a bus, e.g., at the portion 350 of bus
346 described with respect to FIGS. 13 and 14.
Surface 360 comprises a plurality of raised portions 364 in a
concentric arrangement, forming receiving areas 366 between
portions 364. In a compression system into which pressure pad(s)
are incorporated, the pressure pads are typically preloaded such
that the equilibrium stress level (stress level at rest)
counteracts stresses arising from the pressurization levels of the
working fluids of the electrochemical cell. In other systems,
additional pressure (e.g., approximately 50 psi) is added to ensure
contact between the cell parts. Typically, during operation,
compression members 356 are maintained at compressive stress levels
between about 50 psi to about 2500 psi, preferably between about
375 psi to about 500 psi. The configuration of surface 360 allows
the elastomeric material to spread in the radial direction into
areas 366 upon compression portions 364 of in the axial
direction.
In a further embodiment, compression members 356 are configured
with a portion 358 at the circumferential edge that allows terminal
316 to extend through the inside of shim 320 through endplate 304.
Alternatively, the pressure pad is configured such that the
terminal passes through an opening in the pressure pad. The
dimensions are such that upon assembly, there is minimal space
between one face of terminal 316 and portion 358, and the opposite
face of terminal 316 and the inside edge of shim 320. In another
embodiment, portion 358 is configured to facilitate formation of a
sub-assembly, for example as was described above with respect to
portion 354 of terminal 316.
Referring now FIG. 18, a view of cell stack 300 is provided
indicating the components therein. The number of electrochemical
cells 302 employed vary depending on factors including but not
limited to space requirements, hydrogen gas demand, available
electrical energy, desired voltage loss, economy, and selected
materials of construction. As few as one cell 302 is employed as
shown in FIG. 3. As many cells 302 as necessary to effectively
conduct electricity through the stack can be connected. For
purposes of clarity, only one cell assembly 302 is illustrated in
FIG. 12 in exploded form. The additional cells are only
representatively illustrated.
Each cell 302 is disposed between separators 368 to prevent fluid
communication between cells. Separators 368 are electrically
conductive. In one embodiment, separators 368 are titanium sheets
between 0.005 and 0.010 inches thick. Other suitable conductive
materials for the separators include, but are not limited to,
titanium, zirconium, platinum, or palladium.
Each cell 302 includes a membrane electrode assembly 370 having an
oxygen electrode (e.g., an anode) and a hydrogen electrode (e.g., a
cathode) disposed on opposite sides thereof as described above.
Each cell 302 further includes flow fields defined generally by the
regions of fluid flow on each side of membrane electrode assembly
370. Thus, on the side of the oxygen electrode, a flow field is
created within a frame 372, and on the side of the hydrogen
electrode, a flow field is created within a frame 378. Frames 372,
378 are generally formed of plastic material and include suitable
passages for fluid flow and openings for structural support.
Suitable plastics for frames 372, 378 include, but are not limited
to polyetherimides (e.g. ULTEM.RTM. 1000 commercially available
from General Electric Company, Pittsfield, Mass.), polycarbonates,
polysulfonates, or any blend or mixture of any of the
aforementioned plastics.
An optional gasket 376 (e.g., formed of materials similar to those
described above with respect to gasket 324) is disposed between
frame 372 and separator 368 generally for enhancing the seal within
the reaction chamber defined on the oxygen side of cell 302 by
frame 372, separator 368, and the oxygen electrode of membrane
electrode assembly 370.
Frames 372, 378 typically surround (in the radial direction) for
example a screen pack that aids in support of MEA 370. The screen
pack 380 is disposed in the flow field between the hydrogen
electrode of membrane electrode assembly 370 and separator 368, in
fluid communication with the hydrogen electrode of membrane
electrode assembly 370. An optional gasket 382 is disposed between
frame 378 and separator 368 generally for enhancing the seal within
the reaction chamber defined on the hydrogen side of cell 302 by
frame 378, separator 368, and the hydrogen electrode of membrane
electrode assembly 370.
Referring now to FIGS. 19-20, a portion of frame 378 is
illustrated. Frame 378 includes ridges disposed in a surface of
frame 378 proximate to the fluid passages. By way of example,
ridges are molded, cut, or otherwise formed within frame 372. Frame
372 and/or 378 is provided having one or more gaps 384 between the
inside edge and the contents within the flow field. One or more
gaps similar to gap 384 are preferably included within the inside
edge of the frame in fluid communication with water ports 386 and
388 within the frame. Further, while frame 372 and/or 378
illustrated in FIGS. 19-20 include the ridges, such ridges are not
necessary.
Gaps 384 are configured proximate to the water ports (both the
ports 386 and 388 traversing axially through a plurality of
components and a plurality of manifolds 390 traversing radially
from an opening that is part of the axially traversing port to the
flow field within the frame 372 and/or 378). Gaps 384 may further
be configured to be in fluid communication with ports 386 and 388
via the plurality of manifolds 390.
Gaps 384 enhance even water distribution across the contents
allowing a more distributed water flow across the flow field (and
the active area of the cell membrane 370). The dimensions of gap
384 vary depending on the desired flow. Preferably, the dimensions
are optimized to balance the improved flow with the required
support imparted by the frame inside edge.
In one embodiment, port 388 is in fluid communication with the
water intake and port 386 is in fluid communication with the water
and oxygen that exit the cell (e.g., port 388 feeds process water
102 (FIG. 1), and oxygen gas 104 and process water 108 (FIG. 1) are
discharged via port 386). A third port 389 is configured to receive
output hydrogen gas and to effectively channel the hydrogen gas to
fitting 314, as was illustrated with respect to FIG. 1. Use of
three ports (in contrast to the four ports commonly seen in the
prior art) unexpectedly provides adequate flow of fluids and gases,
while at the same time decreases cost of manufacture, and improves
resistance to leakage, particularly at high operating
pressures.
Referring particularly to FIG. 20, gap 384 is provided along a
portion of the height of the inside edge of the frame 372 and/or
378. A protector lip 392 is also provided within the frames 372
and/or 378, generally for preventing the membrane from extruding
into the frame contents (e.g., the screen pack). Protector lip 392
is integral with the frame (e.g., molded integrally or machined
from the same component as the frame).
In yet another alternative, a gap is created that serves the same
or similar function as gap 384 by appropriately formed edges of the
contents of the flow field, e.g., within the screen pack.
While the gaps are described herein as being in fluid communication
with the water manifolds, it is further contemplated that gaps
similar to gaps 384 be employed for enhanced gas distribution, for
example in fuel cell operation.
Advantages of the present invention include lower electrical
resistance thereby leading to higher current densities, simplicity
of assembly and preparation, lower overall cell stack cost,
increased reliability, increased cell life, and decreased space
requirements.
While preferred embodiments have been shown and described, various
modifications and substitutions may be made thereto without
departing from the spirit and scope of the invention. Accordingly,
it is to be understood that the present invention has been
described by way of illustrations and not limitation.
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